The development of optoelectronic devices using organic and/or polymeric molecules is one of the most significant challenges of 21st century electronics [1,2]. Such devices would be inexpensive, easy to handle and, at the same time, would permit size flexibility in a wide scale. The wavelength range of sensitivity of these devices is currently the subject of extensive study. On the other hand, thermo-sensitive organic materials, serving as the active component of thermistors, have also been introduced [3].
Similarly, the development of organic photovoltaic cells is a field which holds much promise of future practical application. The idea of producing photoconductive systems by direct irradiation at the charge-transfer absorption band of donor/acceptor organic molecules has been under discussion since the early 1960's. One of the first systems studied was a photoconductive charge-transfer (CT) complex of tetracyanoethylene (TCE) in tetrahydrofuran (THF) [1]. The conductivity of the system was found to be due to the dissociation of the charge-transfer triplet state of the TCE/THF complex. A few years later, a polymer complex: poly(N-vinylcarbazole) and o-dinitrobenzene in the presence of a strong Brønsted acid (trichloroacetic acid), was also shown to be photoconductive [2]. The formation of the conductive states occurred via a bimolecular reaction between an excited state of the carbazole-nitroaromatic charge transfer complex and a proton.
Recently, an effective photoconductive polymeric gel, based on the pyridine molecule, i.e. poly(4-vinyl pyridine (P(4VP)) in liquid pyridine (Py), molar ratio 1:1, was described [4]. The P(4VP)/Py gel is basic with a pH of 9.1. Photoconductivity was observed in the gel under irradiation at 385 nm at the proton transfer center N+H, which formed on the polymer side chain. The conductivity was explained as being due to the proton mobility in the excited state and to the conjugated structure of the protonated species [5,6]. The changes in conductivity are proportional to the square root of the light intensity with deviation in the high and low intensity range. The deviation in the low intensity range was associated to the activation energy of the process and in the high intensity range, to the limited concentration of the proton transfer centers.